14 SPORES AND THALLIDIA. Horse-tails exhibit a process of spore-formation quite peculiar to themselves. Two species of this group-namely, Eqwisetum, arvense and E. sylvaticum^ are shown in figs. 190 ^ and 1901 At the top of the hollow stem there is a spike ot peltate scales borne on short stalks and arranged in whorls, each of which must m consideration of its origin, be looked upon as a metamorphosed leaf {of fig. 190 ). Kg. 190.—Horse-tails. 1 Summer Shoot of Equisetmn arvense. 2 Vernal fertile Slioot of JEquisetum arvense. ' Spilie ot whorled sporangiophores from tlie same Equisetwm. * A single sporangiophore. 6, 6 Spores. ? Equisetwm sylvatvywm. 8 Prothallium of a Horse¬ tail. 1,2,' natural size; s x 3; < X 6; 6, « x 26; « x 30. On the inner surfaces of the scales^i.e. those turned towards the axis of the spike— little warts arise, which develop into sporangia {cf fig. 190*). The outer cell-layers of these multicellular warts become the walls of the sporangia, whilst the inner tissue breaks up into cells. These cells then divide into four cells, each of which becomes a spore.